1. Field of the Invention
This invention relates to continuous stream, pagewidth ink jet printing and, more particularly, relates to an ink jet printer having means for correcting the droplet trajectories to account for variations in the droplet throw distance, thus improving droplet placement accuracy.
2. Description of the Prior Art
As is known in the art, ink jet printing is a form of non-impact printing wherein ink droplets are caused to impinge upon a recording medium, such as, for example, paper or the like. Ink jet printing is generally categorized as drop-on-demand or continuous stream. In drop-on-demand systems, a droplet is expelled by a droplet generator only when a droplet is required to build information on the recording medium. The continuous stream type systems continually emit streams of droplets. The droplets not required to print information on the recording medium are directed to a gutter, whereat the unused droplets are collected and reused.
Within the continuous stream type of ink jet printer, there exists two basic architectures. One comprises a droplet generator having one or more nozzles which traverse back and forth across the recording medium. The other basic architecture includes a fixed array of nozzles, each of which direct ink droplets to only selected portions of a moving recording medium.
In continuous stream, pagewidth printing, a lineal array of fixed nozzles are positioned transverse to the direction of a moving recording medium and each nozzle directs a stream of ink towards the recording medium. The ink from the nozzles is under a predetermined pressure and is perturbed at a predetermined frequency, so that the streams break into droplets at the approximate same fixed distance from their respective nozzles and, once into droplets, travel at about the same velocity. Each nozzle is assigned printing responsibility for a lineal segment, the total number of lineal segments produce a line across the width of the recording medium. To cause the droplets of each nozzle to fan out across its lineal segment, they are charged by a charging electrode at the breakoff point of the ink stream according to digitized data signals and the charged droplets are passed through an electric field. Those droplets that are not to be printed are directed to a gutter for collection and recirculation to the ink supply for reuse.
Each of the multiple ink jet nozzles of the continuous stream, pagewidth printing architecture throws droplets to specific locations along its lineal segments. When such an ink jet printer is functioning properly, the ink droplets from adjacent nozzles targeted for respective confronting end locations on their adjacent lineal segments "stitch" together without unwanted overlap or without out-of-tolerance gaps therebetween. Further details regarding this type of ink jet printer can be obtained, for example, by reference to U.S. Pat. No. 4,238,804 to Warren.
As droplets from each nozzle are generated and deflected along various trajectories to specific locations within their assigned lineal segment, there is a need to monitor and to correct the performance of the ink jet printer components such as the droplet generator, charging electrode and deflection field so that calibration of the printer does not deteriorate. One important calibration check is that of the stitching point between lineal segments printed by droplets from adjacent nozzles. Neither droplet overlap or gaps between lineal segments can be permitted, if the ink jet printed image is to be uninterrupted across the full width of the recording medium. It is known from U.S. Pat. No. 4,255,754 to Crean et al, for example, to place a sensor at locations representing each end of each lineal segment to optically sense the droplets passing thereby and then determine the precise position of those droplets. This information is used to monitor and correct the charges to be placed on subsequent droplets issuing from each nozzle in order to accurately direct the droplets to be printed to their designated impact or pixel locations on the recording medium.
Research Disclosure 20123, January 1981, by S. C. Paranjpe discloses a correction method for misdirected droplets caused by, for example, manufacturing defects and dimensional tolerance variations. Printing errors are corrected by adjusting the charge voltage prior to subjecting the droplets to the deflection field. A correction alogrithm for each ink jet stream may be developled, and, if desired, the algorithm can be altered over the life of the printhead, as the misalignment of jets produced by the printhead gradually changes.
IBM Technical Disclosure Bulletin, Vol. 22, No. 7, December 1979 by J. R. Booth et al discloses a technique for correcting the fight time of droplets from a reciprocating, ink jet printhead to a fixed, but steppable recording medium to compensate for the impact position error caused by the movement of the printhead relative to the recording medium during the droplet flight time from the printhead to the recording medium.
U.S. Pat. No. 4,136,345 to M. H. Neville et al discloses several height sensing techniques for detecting the deviation of the height of a vertically fanned sequence of droplets from a predetermined flight path and correcting the deviation in subsequence droplets. In one technique, the droplet velocity is determined and adjusted to achieve the desired flight path by increasing or decreasing the nozzle pressure.
U.S. Pat. No. 4,158,204 to L. Kuhn et al discloses a system to correct velocity variations between a plurality of ink jet streams caused by such items as nozzle imperfections, clearances, accumulations and deposits of ink and the like. The velocity compensations between streams of droplets may be made by adjusting the time at which information imparting signals are applied to the respective droplet charging electrodes.
U.S. Pat. No. 3,864,692 to J. A. McDonnell et al discloses a system for controlling ink droplet flight paths by varying the time that voltage is applied to the deflection electrodes to impart to each droplet in a sequence of droplets a different trajectory according to the time each droplet is subjected to the deflection force.
U.S. Pat. No. 4,138,688 to R. S. Heard et al discloses a system for controlling the flight paths of the ink droplets. To compensate for the droplet placement error caused by movement of a printhead relative to the recording medium, a voltage gradient is applied across at least one of the deflection electrodes so as to effect electric field distortion to thereby compensate for the droplet misalignment due to the printhead motion. The amount of distortion is controlled by monitoring the printhead velocity and automatically feeding back a signal to the circuitry controlling the distortion of the electric field between the deflection electrodes.
None of the prior art above recognizes or addresses the problem of stitch point error in a multiple nozzle pagewidth printer caused by variation in the throw distance from nozzle to nozzle to the recording medium, such error, being generated, for example, by variation in recording medium thickness, slight curling or wrinkling of the recording medium and throw distance variations caused by recording medium transport or platen tolerances.